iMovie won't move back to top level photo library
So I'm trying to create my first ever movie in iMovie, and it's not very intuitive. I selected a photo from my Photos library via iMovie's My Media area and dragged it onto the timeline, but now that I've selected a specific photo, I cannot move back up to the top level album the photo is from to choose another photo. When I click on Photos Library, it shows me only the photo I've previously selected, and there's no options to move back to the top level. At least, not that I'm able to find, and nothing's leaping out at me.
Yes, I KNOW I can drag/drop photos to iMovie to create a whole new library, but why duplicate things when iMovie is able to access my Photos library? How can I move back to the top level of the albums and choose another album & picture from it? Help shows nothing. Multiple searches on Google only show how to choose an iPhotos library, which does no good & doesn't do anything in iMovie.
This is all iMovie shows, that one cat picture, & no available way to move back up to the top-level Album it's contained in. iMovie showed the entire album & all the pictures before, right up until I selected this particular picture, and now there's no way to move back:
MacBook Pro (Retina, 13-inch,Early 2015), macOS High Sierra (10.13.3), 16GB RAM, 3.1ghz intel core